This invention concerns improvements in or relating to aircraft gas turbine engines, and also a method of starting aircraft gas turbine engines.
The disclosure of U.S. Pat. No. 5,694,765 is incorporated herein by reference.
In modern aircraft there is generally an increasing requirement for electric power. Such increased electric power may be required for additional cabin services and particularly for use in providing the cabin air supply. FIGS. 2 and 3 diagrammatically show a conventional arrangement to allow various accessories to drive or be driven by the engine of an aircraft.
In more detail, FIGS. 2 and 3 of the drawings show an external gearbox 110 which connects to a starter motor 112 and first and second hydraulic pumps 114, 116. The gearbox 110 connects via a gearbox drive shaft 118 bevelled gears 120 to an angled drive shaft 122. The drive shaft 122 connects via gears 124 to a radial drive shaft 126. The drive shaft 126 connects via bevelled gears 128 to the high pressure compressor 130. The intermediate pressure compressor 132 is also shown.
In use, to start an engine the starter motor 112 is powered which drives the high pressure compressor 130 through the arrangement described above. Once the engine is running the high pressure compressor 130 will drive the gearbox 110 and the attached units including hydraulic pumps 114 and 116.
As indicated above, modern aircraft generally require an increased power to be extracted by the engine to drive accessories. With the above described arrangement, at the top of descent when the engine power is reduced, the large power offtake requirement would lead to an engine surge. This problem could be solved by driving the gearbox from the intermediate pressure compressor rather than the high pressure compressor to ensure that the compressors do not surge at the top of descent conditions.
This would however lead to other issues in starting the engine. With such an alternative arrangement during a start the starter motor would drive the intermediate pressure compressor rather than the high pressure compressor. This would require a very long time to start the engine if the engine would in fact start, as the high pressure compressor would only be induced to rotate by airflow, and would not be positively driven. It therefore is necessary to positively drive the high pressure compressor.